A low-pass filter passes frequencies below its cutoff and attenuates those above it

A low-pass filter (LPF) divides the frequency spectrum at a cutoff frequency: signals below the cutoff pass through unaltered, while signals above the cutoff are progressively attenuated. On a harmonically rich waveform, reducing the cutoff removes upper harmonics, making the sound darker or more muffled. Raising the cutoff restores brightness by letting more harmonics through. The transition is not a brick wall — the steepness of the rolloff is set by the filter slope (measured in dB per octave). The low-pass filter is the most common module in subtractive synthesis: starting from a waveform rich in harmonics, the filter sculpts timbre by subtracting content.

Examples

Patch a sawtooth into an LPF and slowly reduce the cutoff; observe the upper harmonics disappearing on a spectrum analyser as the sound darkens. In Strudel, add a low-pass filter to a drone: note('c3').lpf(500) vs note('c3').lpf(200).

Assessment

Given a sawtooth through an LPF at 800 Hz, predict what happens to the sound if you raise the cutoff to 5 kHz and if you drop it to 200 Hz. Which harmonics are affected in each case?